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Antibody-mediated immunity involves the activation of B cells, which, when activated, begin producing vast quantities of antibody. The types of antibody the cells produce typically depends on what type of pathogen has caused the infection, and is produced in response to various cellular and chemical signals received from other cells and molecules in the immune system.
There are five different isotypes of antibody, each with specific roles in the immune response.
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This is the principle antibody found in blood and body fluids. Around 75% of the antibody circulating in the blood is IgG, and it is this isotype which provides the majority of the antibody-mediated protection against infection.
IgG is rarely produced during an initial response to a given pathogen: this antibody is not produced until around one month initial B cell activation. The principle function of IgG is to bind to pathogens via an antigen-specific receptor, and form antigen-antibody complexes. The formation of these complexes targets the pathogen for destruction via other immune cells.
Note – IgA and IgE have similar functions. All three antibodies have a receptor called the Fc receptor, which interacts with other immune cells such as phagocytes and granulocytes. When this interaction occurs, the cells are triggered to destroy the antibody-antigen complexes, thus also destroying pathogens or their toxic secretions.
IgG also activates the complement system, a cascade of reactions between a set of proteins which results in the formation of a molecule capable of destroying bacterial cells.
In addition, IgG is the only antibody which is able to cross the placental barrier, thus providing passive immunity to a developing fetus.
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IgA is involved in mucosal immunity, preventing colonization by bacteria in areas which are otherwise unprotected from pathogenic attack. This antibody is found in mucosal areas – the gut, the respiratory tract, and the urogenital tract.
In addition IgA is secreted in tears, saliva, and breast milk. In breast milk, IgA provides nursing infants with passive immunity against pathogens the mother has encountered (as opposed to the active immunity the infant will acquire when its own immune system is activated).
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This isotype plays an important role in immune responses to certain parasites, in particular parasitic worms.
IgE is also strongly associated with allergic reactions, due to its ability to trigger granulocytes to release their toxic chemicals when the antibody comes into contact with its specific antigen. These antigens are proteins from the animal and plant species which commonly trigger allergies – dust mites, pollen, cat dander, and others.
Strong allergic reactions can trigger massive inflammation as well as other associated symptoms, such as skin rashes and mucus secretion. The reason why some people experience strong allergic reactions and others do not is not well understood. Twin studies have shown that in the case of asthma, at least, the answer lies in a mixture of genetics and environment.
Recent research also suggests that IgE plays a role in the immune system’s response to cancer cells, although this role has not yet been explored in-depth.
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The function of IgD is not particularly well-defined. This antibody appears as an antigen receptor on naïve B cells (those which have not yet been activated by antigen), and provides a signal to B cells at the end of their maturation process in the spleen. Apart from this, IgD has no other known role.
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The IgM isotype is expressed on the surface of B cells, but it is also secreted by cells. IgM is typically involved early in an immune response, before B cells have had time to begin secreting large quantities of IgG.
This isotype can also trigger the complement cascade, via the same pathway as IgG. However, IgM is a more efficient complement trigger: a single IgM-antigen complex can trigger the cascade, whereas multiple IgG-antigen complexes are required.
Types and Functions of Antibodies in Humans
This five-part series looks at different types of immune responses, including innate and adaptive, and cell-mediated and antibody-mediated responses.